1. Field of the Invention
This invention relates to apparatus incorporating solid state inverters for providing compensation on a three-phase electric power line. More particularly, it relates to apparatus which injects a compensating voltage into the power line in series, such as for example, a dynamic voltage restorer or an active power line conditioner. An inverter forms a load side neutral for the secondary windings of a transformer having the primary connected to the source end of the line.
2. Background Information
There are various types of apparatus for providing compensation on electric power lines which incorporate power inverters. One such system is the Dynamic Voltage Restorer (DVR) an example of which is described in U.S. Pat. No. 5,329,222. The DVR injects a voltage in series with a utility source for the purpose of making up missing voltage that occurs when upstream faults cause short term voltage sags. The DVR described in the above mentioned patent uses three single-phase inverter modules with a common dc link which are connected to the utility line through injection transformers. The common dc link approach allows the use of one common source of energy to supply three separate injection systems, one for each phase, in a three-phase system. This common source of energy can be for instance, a capacitor bank or an interface to draw the required power from the line, even when the voltage is sagged. The injection transformers isolate the power electronics from the power line allowing the maximum injection capacity to be less than the source voltage. It has been shown that the vast majority of sags in utility systems are less than 50% of the phase-to-neutral voltage. The DVR can correct these sags with an inverter rating of less than the connected downstream load which is protected by the DVR.
In practice, the inverter modules of the DVR can be implemented as individual power devices, if the ratings are high enough; series strings of power devices as is the present practice and described in U.S. Pat. No. 5,347,166; or single-phase inverter modules as described in U.S. Pat. No. 4,674,024.
As previously stated, the topologies which have an injection transformer allow the use of a common dc link and energy storage interface since the injection transformer provides the required isolation. The injection transformer also allows a particular power rating to be adapted to many different application voltages with only a change in the transformer and some sensing devices. The major problem with the injection transformer approach is the requirement of a transient flux capability of twice the rated injection voltage of the system, if arbitrary waveforms are to be produced. This requirement makes the transformer larger and unusual, and thus more expensive. Other accuracy and bandwidth limitation problems are introduced by transformer leakage reactance. Unavoidable dc offsets in the inverter make it difficult to prevent saturation, even if the injection transformer has a large flux margin. When the injection transformer saturates, large currents can flow in the inverter if fast control action is not taken. This requires a fast current limit or a trip to occur.
A system without an injection transformer would avoid some of these problems, but prevents the use of a common dc link. Such a requirement for separate energy source/supply interfaces for the inverter for each phase increases the cost and complexity of the system. Some flexibility of manufacture is also lost since the inverter voltage rating must be changed for different system voltages. More devices must be seriesed in a string inverter or more single-phase modules must be connected in series for the modular approach as the voltage increases, assuming the same power devices are used. Differently rated power devices can also be substituted, but this requires more development and requires a more varied parts inventory with lower quantities of an individual power device type.
Another type of compensation equipment which uses series injection with transformer coupling is the active power line conditioner (APLC). The APLC includes a series inverter and a parallel inverter connected by a common dc link. The parallel inverter transfers real power to and from the series inverter and also supplies harmonic currents and the reactive component of the load current. The series path in the APLC suffers from the same limitations as the DVR due to the series injection transformer discussed above.
There is a need, therefore, for an improved apparatus for providing compensation in electric power lines.
More particularly, there is a need for improved apparatus for providing compensation in the form of a voltage injected in series with the power line.
There is an additional need for such apparatus which avoids the prior art problems associated with saturation of an injection transformer.